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Feasibility of Ultraviolet Light Emitting Diodes as an Alternative Light Source 
for Photocatalysis 


3 Lanfang H. Levine and Jeffrey T. Richards 

4 Dynamac Corporation, Kennedy Space Center, FL 

5 Robert Soler and Fred Maxik 

6 Lighting Science Group Corporation, Satellite Beach, FL 

7 Janelle Coutts 

8 Department of Chemistry, University of Central Florida, Orlando, FL 

9 Raymond M. Wheeler 

10 Engineering Directorate (NE-S), Kennedy Space Center, FL 

1 1 ABSTRACT 

12 The objective of this study was to determine whether ultraviolet light emitting diodes 

13 (UV-LEDs) could serve as an alternative photon source efficiently for heterogeneous 

14 photocatalytic oxidation (PCO). An LED module consisting of 12 high-power UV-A 

15 LEDs was designed to be interchangeable with a UV-A fluorescent black light blue 

16 (BLB) lamp in a Silica-Titania Composite (STC) packed bed annular reactor. Lighting 

17 and thermal properties were characterized to assess the uniformity and total irradiant 

18 output. A forward current of (If) 100 mA delivered an average irradiance of 4.0 mW 

19 cm' 2 , which is equivalent to the maximum output of the BLB, but the irradiance of the 

20 LED module was less uniform than that of the BLB. The LED- and BLB-reactors were 

21 tested for the oxidization of 50 ppm v ethanol in a continuous flow-through mode with 

22 0.94 sec space time. At the same irradiance, the UV-A LED reactor resulted in a lower 

23 PCO rate constant than the UV-A BLB reactor (19.8 vs. 28.6 nM CO 2 sec' 1 ), and 

24 consequently lower ethanol removal (80% vs. 91%) and mineralization efficiency (28% 

25 vs. 44%). Ethanol mineralization increased in direct proportion to the irradiance at the 

26 catalyst surface. This result suggests that reduced ethanol mineralization in the LED- 

27 reactor could be traced to uneven irradiance over the photocatalyst, leaving a portion of 


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28 the catalyst was under-irradiated. The potential of UV-A LEDs may be fully realized by 

29 optimizing the light distribution over the catalyst and utilizing their instantaneous “on” 

30 and “off’ feature for periodic irradiation. Nevertheless, the current UV-A LED module 

3 1 had the same wall plug efficiency (WPE) of 1 3% as that of the UV-A BLB. These results 

32 demonstrated that UV-A LEDs are a viable photon source both in terms of WPE and 

33 PCO efficiency. 

34 

35 IMPLICATIONS 

36 Hg-vapor lamps are common UV sources for photocatalysis but create safety and environmental 

37 concerns because they contain Hg; furthermore they have a relatively short life span. This paper 

38 demonstrated that the UV-A LED is a viable alternative to the Hg-vapor lamps without 

39 sacrificing PCO efficiency if the design of the LED arrays is improved to increase the irradiant 

40 uniformity. The use of LEDs could eliminate hazardous Hg wastes and extend photocatalysis 

41 application to places requiring more compact and robust air purification solutions. 

42 INTRODUCTION 

43 The ability of titanium dioxide (Ti02)-assisted photocatalytic oxidation (PCO) to decompose 

44 (mineralize) a broad range of organic contaminants into CO 2 and H 2 O at room temperature has 

45 attracted attention for various environmental applications. This technique has been investigated 

46 as an alternative or complimentary method for air contaminant control 1 ' 7 as well as a means for 

47 treating water and wastewater. 7 The Ti02-catalyzed PCO process typically requires a light source 

48 with a wavelength less than 388 nm. Mercury vapor lamps such as UV-A black lights and UV-C 

49 germicidal lamps have been widely used in laboratory and commercial PCO systems (e.g. 

50 GENESIS AIR PHOTOCATALYST GAP™, Ultra Sun Technologies, and Mazyck 

51 Technologies). But these lamps contain trace amounts of toxic mercury, there are fragile, and 

52 have a relatively short life span (<12,000 hrs). Mercury is a highly toxic and controlled 

53 substance, and it is increasingly becoming controlled or banned by government safety and 

54 environmental regulators. Although there are non-mercury lamps, e.g., microwave generated 

55 UV sources, the lamps are driven by magnetrons that must withstand long duty cycles and the 

56 microwaves must be contained for safety purpose. 

57 


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58 On the other hand, light emitting diodes (LEDs, semiconductor-based lighting devices) are 

59 compact, reliable, and long lasting devices. LEDs are driven by direct current, can accommodate 

60 faster switching, and do not contain toxic Hg. They are entering the market of various 

61 illumination applications with an unprecedented speed. Since the development of first 

62 commercial visible LEDs in 1968, the light output from a single device has increased by a factor 

63 of 20 per decade, while the price in US dollar per lumen has declined by a factor of 10 per 

64 decade. 8 White-light LEDs are now surpassing the efficiency of linear fluorescent and compact 

65 fluorescent lamps. 8 Ultraviolet light emitting diodes (UV-LEDs) have been commercially 

66 available since 2003. Currently, UV-A LEDs have a life expectancy of 50,000 hrs at L50, about 

67 5 times that of Hg-vapor lamps. Naturally they have been considered as an alternative light 

68 source for photocatalysis for both gaseous 9 ’ 10 and aqueous applications. 1 1,12 However, most of the 

69 PCO-studies to date were conducted with low-power LEDs of varied wavelengths including 

70 those outside of the TiC >2 action spectrum (e.g. 395 and 405 nm 11,12 ) in different reactors. Chen 

71 et al. studied photocatalytic degradation of percholoroethylene (PCE) in a rectangular steel gas- 

72 phase reactor irradiated by 375 nm UV LEDs (16 Nichia LEDs with 1.0 mW power output) and 

73 found only 43% degradation of PCE in the LED reactor, while there was 90% degradation in a 

74 UV-A black light reactor. 9 This result seemed to imply that LEDs are less effective than the 

75 black light. Ciambelli et al. investigated the photocatalytic breakdown of benzene in a lab scale 

76 fluidized bed reactor irradiated by two or four UV (365-nm) LED modules (Nichia Corporation) 

77 and showed a 27% conversion of benzene into CO 2 at 80 °C. 10 Although these studies proved 

78 that UV LEDs have promise as a light source, no data were provided regarding their PCO 

79 performance relative to mercury-vapor lamps at the same irradiance, their actual power use 

80 efficiency, or issues related to the LED integration into PCO reactors. The objective of this study 

81 was to design an LED PCO reactor and compare the performance of state-of-the art UV-A LEDs 

82 to that of a Hg-vapor UV-A fluorescent black light for low temperature PCO degradation of 

83 organic contaminants. ' 

84 

85 

86 EXPERIMENTAL METHODS 

87 Light Sources 

88 An 8-W UV-A (F8T5) fluorescent black light blue (BLB) lamp from Philips was used as the 

89 control light source. The BLB lamp dimensions were 15.6 mm D x 304.8 mm L and the irradiant 


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90 output was 2.5 mW cm' 2 at a 25.4 mm distance. The LEDs used for the study were high-power 

91 chip-type UV-A LEDs (model NCSU033B) from Nichia Corporation, having a peak wavelength 

92 of 365 nm, a spectrum half-width of 9 nm, and irradiance angle of 120 degrees. The optical 

93 output of a single LED was 325 mW at a forward current of 500 mA and voltage of 3.8 V (i.e., 

94 1.9 Watts). 

95 

96 To conduct direct comparison of PCO efficiency with UV-A BLB, the LED source was designed 

97 based on three criteria: 1) interchangeability with the BLB in the same PCO reactor; 2) similarity 

98 of irradiance profile between the LED module and the BLB (i.e. isotropic); and 3) a wide range 

99 of irradiances, including one equivalent to that of the 8-W BLB. Based on this, we designed an 

100 LED assembly to simulate the geometry of the linear fluorescent bulb. The placement of 

101 individual LEDs was determined by modeling (Figure 1). Uniformity of the irradiance increased 

102 as the distance between LEDs decreased (Figure la). Thus an assembly with densely populated 

103 LEDs (e.g., 15 mm vs. 30 mm spacing between LEDs) would provide more uniform irradiance, 

104 but the power consumption and initial investment (number of the LEDs) would increase 

105 accordingly. In addition, greater numbers of LEDs create thermal management challenge, where 

106 lower operating temperatures are preferred to maintain the long operating life of the LEDs. The 

107 coefficient of temperature increase per unit electric power input is dependent upon the thermal 

108 resistance of the LED system (e.g., Rj a = 35 °C/W with Nichia’s standard circuit board), density 

109 of the LED placement, and factors such as ambient temperature. The radial irradiance profile of 

110 the LED assembly was modeled based on four linear LED arrays evenly placed every 90-degrees 

111 in a 360-degree arrangement. The model examined the effect of viewing angle and distance 

112 between the light source and the object to be irradiated (Figure lb). Since the object to be 

113 irradiated was the photocatalyst packed in an annular reactor (see details in PCO reactor design), 

114 optimal diameter (0) of the quartz sleeve separating the light source and the catalyst was 

1 15 subsequently determined by this modeling exercise. Although increasing the diameter enhanced 

116 the uniformity of light distribution, the radiant flux per unit area (E) decreased approximately 

117 following an inverse-square law (Figure lb). Figure 1 suggests that 20-mm spaces between 

118 LEDs, and a 25-mm diameter would give satisfactory uniformity and sufficient intensity. 

119 Consequently, twelve LEDs were mounted on a 15.6-mm diameter aluminum rod in four linear 

120 arrays, three LEDs in each array with a space of 20 mm between the LEDs as shown in Figure 2. 


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121 


122 Figure 1 and Figure 2 here 

123 c ' Photocatalytic Oxidation (PCO) Reactor 

124 Silica-Titania Composite (STC) pellets (2x6 mm) from Sol-gel Solutions, LLC (Gainesville, 

125 FL) served as the photocatalyst. The STC has the same porosity (30 - 40 A) and Ti02 loading (4 

126 g Degussa P-25 in 100 mL silica precursor, tetraethyl orthosilicate) as that used by Stokke and 

127 Mazyck. 13 An annular reactor shown in Figure 2 was used to carry out this study because of its 

128 simplicity and efficiency in utilizing traditional linear fluorescent lamps. It consisted of two 

129 concentric cylinders, with an annulus formed between an aluminum housing and a quartz sleeve. 

130 The light source was inserted in the middle of the quartz sleeve, while STC pellets were packed 

131 in the annulus. Two design parameters were optimized: 1) the diameter of quartz sleeve that 

132 determines the distance between the photocatalyst and light sources; and 2) the annulus size. 

133 The former was especially critical with LEDs as the light source. The effect of the quartz 

134 sleeve’s diameter was illustrated in Figure lb. The annulus space that determines the thickness 

135 of the catalyst bed should be small enough to ensure that photons emitted from the light source 

136 reach all catalyst surfaces uniformly and large enough to allow reproducible packing of STC 

137 pellets. Results of a preliminary light transmittance measurement showed that STC pellets in an 

138 annulus of 8 mm used by Stokke and Mazyck 13 attenuated 97% of a UV-A BLB light. This 

139 suggested the annulus size of the reactor should be further reduced. Key parameters of the bench- 

140 scale test reactor used in this study and that used by Stokke and Mazyck 13 are listed in Table 1 

141 for comparison. 

142 

143 Table 1 here 

144 Light Source Characterization 

145 Spectral quality and quantity of the light sources were assessed to determine an optimal UV-A 

146 BLB for the study and the driving current required for the LED assembly to achieve an 

147 equivalent irradiance. Measurements were conducted outside the PCO reactor in a dark room 

148 using a spectroradiometer (OL754C, Optronics Laboratories, Orlando, FL). The light source 

149 (either BLB or LED assembly) was centered inside a quartz sleeve of the same dimension as that 

150 used in the reactor (Table 1) without the catalyst around it and placed directly above the 


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151 integrating sphere of the spectroradiometer. A light attenuation aperture of 12.7-mm in diameter 

152 was placed before the integrating sphere that has an opening of 31.8 mm in diameter. This setup 

153 (Figure 3) measured the irradiance immediately at the surface of the catalyst bed. For the LED 

154 module, measurements were taken every 5 mm along the lateral direction both directly opposite 

155 to one of four LED arrays (designated as angle 0 degree) as shown in Figure 3 and between two 

156 linear LED arrays (designated as angle 45 degree). Irradiant output of the LED assembly and 

157 LED die’s temperature were measured at a range of forward current between 30-500 mA. The 

158 BLB was measured at three positions along the axis of the lamp. 

159 

160 Figure 3 here 

161 PCO Tests 

162 Performance of the UV-A BLB- and the LED-irradiated annular reactors was evaluated for 

163 oxidation of ethanol in an experimental setup (Figure 4) that allowed precise control of 

164 experiment variables and continuous monitoring of the PCO reaction. The setup consisted of: 1) 

165 a Kin-Tek air generator (model 49 1M, La Marque, TX) for supplying a simulated contaminant 

166 air containing 50 ppm v ethanol (EtOH) and 72% relative humidity at 25 °C; 2) a PCO reactor 

167 packed with 14.6 g of STC pellets to a bed height of 60 mm; 3) two mass flow controllers for 

168 controlling the flows to the PCO reactor and CO 2 analyzer; 4) temperature sensors for the 

169 reactor’s inlet and outlet as well as for room temperature; 5) humidity sensors for the reactor’s 

170 influent and effluent air; 6) a CO 2 analyzer for the reactor effluent; 7) a sample stream selecting 

171 valve; and 8) a gas chromatograph (ThermoFinnigan, Austin, TX) equipped with a flame 

172 ionization detector (GC/FID) and a HP Plot Q capillary column (30 m x 0.32 mm, 20 pm depth 

173 of film). 

174 

175 Figure 4 here 

176 

177 All tests were carried out in a flow-through mode with an uninterrupted 2 L min' 1 air flow 

178 containing 50 ppm v EtOH under continuous illumination. Each test was repeated three times. 

179 Both influent and effluent were sampled alternately every 8.45 min and analyzed for ethanol and 

180 any oxidation intermediates by GC/FID. The effluent was also directed to a CO 2 analyzer to 

181 determine the production of CO 2 , the complete mineralization product. CO 2 concentration was 


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182 recorded every minute. The reactor was maintained at 25 °C via forced air convection using a 

183 heat sink attached to the PCO reactor. The STC pellets were pre-conditioned with 72% RH, 

184 VOC-free air under continuous illumination. Each test began with the addition of ethanol to the 

185 air stream and continued for 21 hours, followed by regeneration with humidified, VOC-free air 

186 and continuous illumination. The same batch of STC catalyst was used for all runs. Completion 

187 of the regeneration was indicated by no detectable organic species and only baseline-level CO 2 in 

188 the effluent. 

189 

190 PCO Efficiency, Kinetics and Photonic Efficiency 

191 PCO performance was quantified in terms of EtOH removal and mineralization efficiency (Xa). 

192 The former is a measure of the total removal of the test VOC, whether it is removed by 

193 adsorption or oxidation, while the latter is a measure of the complete oxidation of EtOH to CO 2 . 

194 These values were calculated using equations 1 and 2, respectively. Co and Ceioh are the influent 

195 and effluent EtOH concentrations, and AC car bon dioxide is the CO 2 generated from the PCO process. 

196 The rate of photocatalytic oxidation of ethanol was determined based on the formation of CO 2 

197 instead of the disappearance of ethanol to prevent overestimation due to the adsorption of EtOH 

198 to the photocatalyst. Cumulative CO 2 concentration was plotted against time, a linear 

199 relationship between the concentration and time suggested zero-order kinetics. The slope gave 

200 rise to the PCO rate (r). PCO photonic efficiency (^) was calculated as the ratio of the 

201 photocatalytic degradation rate to the incident photon flux (eq 3). 

202 EtOH removal = (Co-CeiohVCo (1) 

203 Xa AC C arbon dioxide /2 X Co (2) 

204 § = Rate of reaction (M sec‘')/rate of photon incident (mol sec" 1 ) (3) 

205 

206 RESULTS AND DISCUSSION 

207 Spectral Quality and Quantity of the Light Sources 

208 Photon flux, or irradiance, on the catalyst surface is one of the most important factors affecting 

209 photocatalytic oxidation efficiency. The LED assembly was extensively characterized in order to 

210 assess its irradiance uniformity and the required driving current for the LED module to provide 

211 an optical output similar to that of an 8-W BLB. Initial scans of four UV-A fluorescent black 


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212 lights from GE, Eiko, Philips, and Sylvania demonstrated that the GE and Eiko lamps are similar 

213 in their spectra and intensity. The Philips lamp ranked the highest in irradiance among the four 

214 lamps examined, while the Sylvania lamp had the lowest irradiance and a very broad peak. 

215 Hence, the Philips brand lamp was used in this study. Relative to the UV-A LED, the UV-A 

216 BLB had a broader peak (354-388 nm) centered at 365 nm and an additional peak at 405 nm that 

217 is out of the TiC >2 action spectrum (<388 nm) (Figure 5). The LED spectrum peak was narrower 

218 (357-378 nm) and all of the radiation fell within the Tick’s action spectrum. Furthermore, the 

219 spectra of adjacent LED linear arrays (LED Array 1 and Array 2) were identical (Figure 5). 

220 

221 Figure 5 here 

222 

223 Lateral irradiance profile of the LED assembly was measured every 5 mm from the first LED 

224 along the lateral axis directly opposite to one of the 4 arrays (angle 0 degree) as well as opposite 

225 to the space between two arrays (angle 45 degree) at the forward current of 100 mA. Irradiance 

226 from the LED assembly was not less uniform than desired (Figure 6). The lowest intensity was 

227 about 55% of the peak intensity, occurring directly between two LEDs in an individual array. 

228 The average irradiance (E) at angle 0 degree was 6.02 mW cm' 2 with 2.49 mW cm’ 2 at angle 45 

229 degree, resulting in a mean of 4.25 mW cm' 2 for the assembly. The overall mean irradiance for 

230 the LED module was 70% of the predicted value (Figure 6b). The discrepancy could be 

231 explained by the directionality (120 degree) of the LED radiation and how the light was 

232 measured (Figure 3). The combination of the small sensor aperture (12.7 mm) and the close 

233 distance (approximately 8 mm) between the sensor and the light source prevented some of the 

234 photons from adjacent LEDs from entering the integrating sphere (Figure 3a). As a result, the 

235 measured value was underestimated comparing with that obtained in the absence of the 

236 attenuation aperture (Figure 3b). Nevertheless, the opacity of STC pellets packed immediately 

237 outside of the quartz sleeve in a working PCO reactor would act as the attenuation aperture and 

238 prevent the photons outside the radius of 12.7 mm aperture from reaching the catalyst located 

239 where the light sensor was placed. We believed that the measured value was a more accurate 

240 representation of the light level the catalyst would intercept rather than the predicted value. In 

241 contrast, the 8-W UV-A BLB lamp from Philips measured in the same way showed a uniform 

242 irradiance of 4.0 ± 0.2 mW cm' 2 along the both radial and lateral axes. 


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243 Figure 6 here 

244 

245 Furthermore, the light output of the LED assembly was also measured at If between 30 to 500 

246 mA. As with the individual LEDs, the irradiance of the LED assembly was directly proportional 

247 to the driving current in the range of 30 to 500 mA (E = 0.0449If-0.2235, R 2 = 0.9999). 

248 Consequently, a nominal 100 mA driving current for the LED assembly delivered the maximal 

249 irradiance of an 8-W fluorescent lamp. 

250 Thermal Characteristics of the LED Assembly 

251 LED life span can vary according to environmental and design related factors. Although it is 

252 largely determined during the engineering phase of an LED lighting design, overdriving an LED 

253 assembly will decrease its life span if thermal management is inadequate. In order to assess the 

254 effectiveness of our heat management strategy and to determine the upper limit of driving current 

255 (and hence the light output) for the assembly, the temperature of each LED in the assembly was 

256 measured at three driving currents (Table 2). The LED temperatures (Tj) were calculated based 

257 on the thermal resistance from the LED die to the measuring point being 7 °C W' 1 . Results 

258 showed that a linear relationship between the driving current and measured solder temperature 

259 (T s ) or calculated junction temperature (Tj), that is, Tj = 0.0857If + 25.4 (R 2 = 0.9999). From 

260 this, the maximal allowable driving current was determined to be 870 mA to operate the LEDs 

261 below the manufacturer’s recommended maximum Tj of 100 °C. Because the LED used in this 

262 study was rated for a maximum forward current 700 mA, the assembly consisting of 12 LEDs 

263 electrically strung in two parallel series should allow for a maximum of 1400 mA and result in a 

264 light output of 62.6 mW cm' 2 based on the established relationship between the irradiance and 

265 forward current (E = 0.0449If-0.2235). It was determined that the LED assembly had a greater 

266 light output potential (62.6 mW cm' 2 ) than that the current thermal management strategy could 

267 deliver (38.9 mW cm' 2 ). That is, from the thermal perspective, the assembly can only fulfill 62% 

268 of its light output potential. This is primarily due to the design constraints for this first 

269 generation LED module to be directly comparable with linear fluorescent lamps. Four linear 

270 LED arrays were mounted on a small aluminum rod; thermal energy (e.g., 12 W at 500 mA 

271 driving current) had to be conducted to the ends for convective dissipation. 

272 

273 Table 2 here 


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274 PCO Efficiency of the BLB and LED-irradiated Reactors 

275 The PCO reactor effluent was found to consist of ethanol (VOC contaminant), acetaldehyde 

276 (oxidation intermediate), and carbon dioxide (final oxidation product). Acetaldehyde (ACD) 

277 was the only quantifiable intermediate in the effluent as indicated by the lack of any other peaks 

278 in the GC chromatograms (data not shown). The UV-A BLB-irradiated reactor generated 

279 effluent ethanol and acetaldehyde profiles (Figure 7b) similar to those reported for methanol 

280 oxidation. 14 Upon the initiation of ethanol-contaminated air flow, effluent ethanol concentration 

281 remained very low (2% of the feed) for the first three hours, increased at an accelerated rate 

282 between 3 and 10 hrs, and continuously crept upwards even after 10 hrs. This initial lag time for 

283 ethanol was attributed to the adsorption of ethanol by STC pellets. In contrast, there was a very 

284 short (less than 30 min) or no initial lag time for ACD and C0 2 , respectively. The concentration 

285 of ACD and C0 2 in the effluent increased steeply upon the addition of feed contaminant, 

286 suggesting low and/or no affinity of ACD and C0 2 to the STC. The concentration of C0 2 

287 approached a plateau or a steady state between 5 and 10 hrs, but that of ACD and ethanol 

288 reached somewhat steady state only after 10 hr. Therefore, the time period between 10 and 20 

289 hrs was considered as the “pseudo-steady state.” The time course profiles of effluent ethanol, 

290 ACD, and C0 2 from the UV-A LED reactor (Figure 7a) resembled those of the BLB reactor in 

291 general shape, but differed in slope and concentration level at the pseudo-steady state. 

292 

293 Figure 7 and Table 3 here 

294 

295 Mineralization of ethanol in both reactors followed zero-order kinetics and had a rate constant of 

296 19.8 and 28.6 nM C0 2 sec' 1 for the LED and BLB, respectively. The average concentration of 

297 effluent components at this time period was used to assess the PCO efficiency in terms of ethanol 

298 removal and mineralization (Table 3). Compared with the UV-A BLB reactor, the LED reactor 

299 had a lower effluent ACD and C0 2 but higher EtOH, which translated into lower EtOH removal, 

300 mineralization, POC rate and photonic efficiency than the UV-A BLB reactor (Figure 8b through 

301 e). The results do not necessarily indicate that the LEDs were a less effective light source, 

302 bearing in mind that the LED module’s irradiance was not as uniform as the BLB; some of the 

303 catalyst was irradiated by less than the average irradiance, which may have accounted for the 

304 reduced C0 2 and ACD in the effluent. 


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305 The effect of irradiance on PCO efficiency was subsequently examined in the LED reactor. 

306 Increasing the irradiance at the catalyst surface reduced effluent ACD and EtOH and increased 

307 CO 2 production (Figure 8a). There was a linear relationship between both the mineralization 

308 efficiency and rate constant and the irradiance (Figure 8c & d). The influence of increasing 

309 irradiance on the percentage of ethanol removal (Figure 8b) was not as pronounced as for the 

310 mineralization. This is not surprising and is attributable to the STC’s unique property of high 

311 physical adsorptivity for polar compounds and photocatalytic activity. However, increasing the 

312 irradiance from 4.0 to 17.7 mW cm" 2 decreased the photonic efficiency (§) by 33%. The linear 

313 relationship between mineralization and irradiance suggested that an irradiance of 7.6 mW cm' 2 

314 from the LED module would reach the same mineralization as that of the BLB at its full 

315 intensity. In other words, the LED reactor used in this study could achieve the same PCO 

316 efficiency for ethanol as an 8-W UV-A FL when it is operated at a forward current (I F ) of 170 

317 mA (i.e., a power input of 3.4 W). These values could be reduced by a more uniform irradiance 

3 1 8 over the entire surface of the photocatalyst. 

319 

320 Figure 8 here 

321 

322 Power Use Efficiency 

323 Power use efficiency of a light source for PCO encompasses both the electric-irradiant 

324 efficiency, or wall plug efficiency (WPE), and the PCO efficiency. WPE (defined as the 

325 percentage of irradiant output per electrical input) of the light sources used in this study is shown 

326 in Table 4. It is apparent that the WPE of the UV-A LED and UV-A FL were comparable, 17% 

327 as the manufacturer specified and 13% as measured in this study. It is interesting to note that the 

328 WPE efficiency is higher for longer wavelength LEDs. For example, the same type of LED with 

329 spectrum centered at 385 nm has a WPE of 21.6%, representing a 25% increase from that of the 

330 365 nm LEDs. However, it is not known whether the gain in WPE would be offset by the 

331 potential loss in PCO efficiency since 385-nm LEDs approach the upper limit of the Ti02 action 

332 spectrum. In terms of PCO efficiency, LEDs in the current design were slightly less efficient 

333 when compared to the UV-A FL, but the gap could be closed if a more uniform irradiance over 

334 the catalyst is achieved. In addition, PCO efficiency of LEDs could be enhanced by exploitation 

335 of its instantaneous “on” and “off feature for periodic irradiation. It was previously 


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336 demonstrated that photonic efficiencies for decomposition of o-cresol by a UV/TiCL process in a 

337 slurry reactor under controlled periodic illumination of LEDs was higher than that under 

338 continuous illumination. 12 As a result, the electric energy required for degradation of the same 

339 amount of contaminants decreased significantly by using periodic irradiation. 

340 

341 Table 4 here 

342 

343 CONCLUSIONS 

344 This is the first report of a direct comparison between UV-A LED and UV-A BLB as PCO light 

345 sources under similar irradiance. Challenges encountered in achieving uniform LED irradiance 

346 over the photocatalyst while maintaining the power use efficiency. Increasing the density of 

347 LEDs could no doubt enhance the uniformity of the irradiance, but it would also increase the 

348 initial cost of a PCO reactor and the burden of heat management if high-power LEDs are used. 

349 The results from our LED reactor suggest that the number of LEDs per unit area may actually be 

350 reduced because of the facts that the LED assembly could deliver up to 38.9 mW cm' 2 and an 

351 irradiance of 17.7 mW cm' 2 resulted in a 97% EtOH removal and an 86% mineralization. It 

352 became clear that different design strategies should be considered depending upon the type of 

353 UV-A LEDs 15 to be used, for instance, a higher density of low-power LEDs (<10 mW) or high- 

354 power LEDs (> 1 00 mW) coupled with light dispersion. Typical approaches for light dispersion 

355 in lab-scale reactors include a) coupling LEDs to light transmitting optical fibers coated with a 

356 thin film of photocatalysts, and b) using a waveguide through which light travels and emits from 

357 sides into surrounding catalysts. Although these approaches have the advantage of transferring 

358 small area of a LED’s illumination to a much greater surface area, each has its own drawbacks. 

359 The former has limited applications to thin films of catalysts because of TiCVs opacity, while the 

360 later creates a gradient of irradiance along the waveguide. A balance between side-emitting and 

361 transmitting must be struck to achieve uniform side emission intensity over reasonable lengths. 

362 We are currently working on screening light conduit materials & LED-light conduit coupling 

363 techniques for effective dispersion of high-power LED’s radiation. 

364 


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365 PCO efficiency in terms of ethanol removal and mineralization was greater in the UV-A BLB 

366 reactor than in the UV-A LED reactor at the same average irradiance (4.0 ± 0.2 mW cm' 2 ). 

367 Irradiance level and uniformity over the catalyst was found to have a great impact on the PCO 

368 efficiency. PCO efficiency increased linearly as the irradiance over the surface of catalyst 

369 increased in the range tested (4-18 mW cm' 2 ). We estimated that the LED reactor used in this 

370 study could achieve the same ethanol mineralization as a 8-W UV-A BLB when it was operated 

371 at forward currents (If) of 170 mA, which corresponded to a power input of 3.4 W and an 

372 irradiant output of 7.6 mW cm' 2 . These values are expected to be lower as uniform irradiance 

373 and/or periodic irradiation are implemented. The results proved that LEDs are a viable photon 

374 source both in terms of PCO efficiency and wall plug efficiency. Continuing efforts in the 

375 following areas will strengthen this conclusion: 1) improvements in the design of the LED-PCO 

376 reactor for a higher fidelity estimate of power use efficiency; 2) investigation of the trade-off 

377 between PCO efficiency and electric-irradiant efficiency by using longer wavelength LEDs (e.g. 

378 385 nm instead of 365 nm); 3) using visible light responsive catalysts to take advantage of the 

379 higher quantum efficiency of longer wavelength LEDs. 

380 ACKNOWLEDGMENTS 

381 The work was conducted under the auspices of Life Science Support Contract and the first part 

382 of a Kennedy Space Center Innovative Partnership Program (IPP) funded project to highlight the 

383 partnership with Lighting Science Groups Corporation (LSGC). Authors are extremely grateful 

384 to Dr. David Mazyck of the University of Florida for donating the photocatalyst. The authors 

385 would like to thank Mr. Lawrence L. Koss for his invaluable assistance with the PCO test bed 

386 construction by making customized parts and Opto 22 data logging. We would also like to 

387 extend our appreciations to Mr. J. Schellack and Mr. D. Johnson of LSGC for constructing the 

388 LED assembly and KSC prototype shop personnel for fabricating the bench scale PCO reactor. 

389 


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418 


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423 2008 , 42(10), 3803-3813. 

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425 

426 About the Authors 

427 Lanfang H. Levine and Jeffrey T. Richards are senior research chemists with Dynamac 

428 Corporation at Kennedy Space Center, FL 32899. Robert Soler and Fred Maxik are both 

429 electrical engineers and serve as director of electrical engineering and chief scientific officer, 

430 respectively for Research and Development of Lighting Science Group Corporation at 1227 S. 

431 Patrick Dr. Satellite Beach, FL 32937. Janelle Coutts is a graduate student from the University of 

432 Central Florida and currently a chemistry intern with Dynamac Corporation at Kennedy Space 

433 Center. Raymond M. Wheeler is a plant scientist with NASA at Kennedy Space Center, FL 

434 32899. Please address correspondence to Lanfang FI. Levine, Space Life Sciences Laboratories, 

435 Mail Code: DYN-3, Kennedy Space Center, FL 32899, USA; phone: 321-861-2931; e-mail: 

436 lanfang. h. levine@nasa. gov . 

437 

438 TABLES 

439 

440 , Table 1 . Comparison of test reactors 


Reactor used in Stokke and 
Mazyck 13 

Annular reactor in this 
study 

Reactor Housing ID (mm) 

41.4 

38.1 

Quartz Sleeve OD (mm) 

25.4 

28.0 

Annulus Space (mm) 

8.0 

5.05 

Catalyst Bed Height (mm) 

35.8,71.5, 107.3 

60 

Bed Volume (cm 3 ) 

30, 60, 90 

31.44 

Temperature 

Not controlled 

Controlled to 25 °C 


15 


441 


442 

443 


444 

445 

446 

447 


448 

449 


450 

451 


Table 2. LED solder temperature (T s ) and dice temperature (Tj) as a function of If 


If (mA) for the LED 

Measured T s 

Predicted Tj 

Assembly 

(°C) 

(°C) 

100 

32.7±0.2 

33.9 

300 

47.8±0.6 

51.3 

500 

62.3±1.0 

68.1 


Table 3. Effluent composition and PCO efficiency at pseudo-steady state from the BLB and 


LED reactors at the same irradiance. Values represent the mean 
Influent ethanol concentration was 5 1 ± 0.3 ppm v . 

(± s.e.) between 10 and 20 hrs. 


UV-A BLB 

UV-A LED 

A 

Average E, mW cm' 

4.0 ±0.2 

4.0 ±0.2 

Effluent EtOH, ppm v 

4.6 ±0.6 

10.5 ±0.2 

Effluent ACD, ppm v 

18.7 ±0.3 

14.2 ±0.4 

Effluent CO 2 , ppm v 

45.5 ±2.5 

28.8 

EtOH Removal (%) 

91.0 ± 1.3 

80.0 ±0.4 

Mineralization (%) 

44.3 ±2.7 

28.2 


Table 4. Wall plug efficiency (WPE) of the light sources 



UV-A BLB 

UV-A LED 
(Individual) 

UV-A LED 
(Assembly) 

Model/Type 

F8T5 

Nichia NCSU033B 

Custom Designed 

Electric Input (W) 

8.0 

1.9 

2.0 

Optical Output (W) Specified 

1.41 

0.33 


WPE (%) Specified 

17.6 

17.1 


Optical Output (W) Measured 

1.06 


0.26 

WPE (%) Measured 

13.2 


13.0 


16 


451 FIGURE CAPTIONS 

452 

453 Figure 1. Effect of LED spacing and distance away from the LED (i.e. Vi 0) on: (a) lateral 

454 irradiance uniformity and (b) radial irradiation uniformity. 

455 

456 Figure 2. A 3-D model of the annular reactor shown with the LED light source. 

457 

458 Figure 3. Schematic of the setup for light source characterization, illustrating the effect of 

459 aperture size on the amount of photons from the adjacent LEDs entering the integrating sphere. 

460 

461 Figure 4. Schematic of a bench-scale PCO test bed where the objects are not to scale. 

462 

463 Figure 5. Spectra of UV-A LEDs and fluorescent black lights. 

464 

465 Figure 6. Irradiance profiles of the LED assembly determined at Ip=100 mA: (a) lateral and 

466 radial profiles on the outer surface of the quartz sleeve (OD 28 mm) where the photocatalyst is 

467 located; (b) comparison between measured average and model-predicted irradiance. 

468 

469 Figure 7. Time-course of the effluent composition during STC-catalyzed oxidation of ethanol 

470 in the (a) UV-A LED and (b) UV-A BLB reactors at the same irradiance of 4 mW cm' 2 . CO 2 

471 concentration was recorded every minute and appears to be affected by the sample stream valve 

472 position giving two parallel trend lines. 

473 

474 Figure 8. Effect of the average irradiance over the catalyst surface on STC-catalyzed PCO in 

475 the LED reactor: (a) reactor effluent composition at the pseudo-steady state, PCO efficiency in 

476 terms of (b) ethanol removal, (c) ethanol mineralization, and (d) PCO rate constant, and (e) 

477 photonic efficiency. 

478 

479 

480 

481 

482 


17 



483 FIGURES 

484 


485 

486 

487 

488 



Figure 1. Effect of LED spacing and distance away from the LED (i.e. 'A 0) on: (a) lateral 
irradiance uniformity and (b) radial irradiation uniformity. 


18 






Aluminum Housing 


Quartz Sleeve 


Air Outlet 


Annular Space 
for Catalysts 


489 

490 


491 Figure 2. A 3-D model of the annular reactor shown with the LED light source. 

492 


493 

494 

495 

496 

497 


(a) 


(b) 




Integrating sphere 
w/ 31.8 mm aperture 


Figure 3. Schematic of the setup for light source characterization, illustrating the effect of 
aperture size on the amount of photons from the adjacent LEDs entering the integrating sphere. 


19 




498 

499 

500 


501 

502 



to GC/FID 


Carbon Dioxide Analyzer 


0.65 psi 56.2% 
PT 2 RH2 


305 

ml/min 


< Viaii OM Mod 


to GC/FID 


GFC 2 
300 mlrmin 


r 

PCO System On 


GFC 1 

2,000 ml/min 


Figure 4. Schematic of a bench-scale PCO test bed where the objects are not to scale. 



503 Figure 5. Spectra of UV-A LEDs and fluorescent black lights. 


20 





504 Distance from one LED to the other (mm) 

505 

506 Figure 6. Irradiance profiles of the LED assembly determined at I F =100 mA: (a) lateral and 

507 radial profiles on the outer surface of the quartz sleeve (OD 28 mm) where the photocatalyst is 

508 located; (b) comparison between measured average and model-predicted irradiance. 


21 




(a) UV-A LED 


61 


(b) UV-A BLB 


• EtOH 


Time of Continuous VOC Feed and Illumination (hr) 


Figure 7. Time-course of the effluent composition during STC-catalyzed oxidation of ethanol 
in the (a) UV-A LED and (b) UV-A BLB reactors at the same irradiance of 4 mW cm" 2 . CO 2 
concentration was recorded every minute and appears to be affected by the sample stream valve 
position giving two parallel trend lines. 




5]5 Irradiance at the Surface of Catalyst (mW cnr 2 ) 

516 

517 Figure 8. Effect of the average irradiance over the catalyst surface on STC-catalyzed PCO in 

518 the LED reactor: (a) reactor effluent composition at the pseudo-steady state, PCO efficiency in 

519 terms of (b) ethanol removal, (c) ethanol mineralization, and (d) PCO rate constant, and (e) 

520 photonic efficiency. 


23